Moving, Without Feet to Do So

Cilia are hairlike structures that project from cells. They can move in coordinated, rhythmic waves that propel organisms through water.

Cilia are hairlike structures that project from cells. They can move in coordinated, rhythmic waves that sweep fluid across the cell surface. If cells are fixed in place, this creates a current of fluid; the cilia that line our respiratory tract remove mucus and debris in this way. If the cilia are on the outside of the organism, the current they create can move the entire organism. This episode of CreatureCast presents two organisms that use cilia for locomotion. One is small and unicellular, the other is a much larger animal.

Stentor is a close relative of Paramecium, and at about 1 millimeter in length, this single-celled organism is barely big enough to spot with the naked eye. Beating cilia propel Stentor as it twists and turns in search of food in freshwater streams and lakes.

Most larger organisms don’t move with cilia, as tiny Stentor does. The many independent cilia just aren’t effective at propelling big things. The ctenophores, also known as comb jellies, are an exception. Mnemiopsis, a ctenophore that grows to be about the size of a small apple, is pelagic: it spends its whole life swimming in the ocean. Mnemiopsis and other ctenophores are the largest organisms to be propelled by cilia. Each cilium doesn’t move independently, as in tiny Stentor. Instead, the cilia of each comb function together as a paddle that is far more efficient at moving such a large organism. The combs, in turn, are organized into rows along the body, and their motion is coordinated across the whole animal.

Image
Credit...Stefan Siebert

The tightly packed cilia within each ctenophore comb are crystal-like. This causes each comb to refract light, splitting white light up into a rainbow of colors. As the combs move at slightly different angles from their neighbors, it appears as if colored lights shoot up and down the sides of the illuminated animal.

My colleagues and I recently published the genome sequence of Mnemiopsis, the first available for a ctenophore. Although ctenophores have complex musculature and nervous systems, we found that they lack many of the genes previously thought to be required for the development and function of these structures. This is consistent with our previous analyses that place ctenophores as our most distant animal relatives.

These organisms were filmed by Stefan Siebert, a postdoctoral researcher in my laboratory, using specimens from the Invertebrate Zoology course I teach at Brown University. He also edited the piece. Bryn Bliska, a teaching assistant in the class, wrote and performed the music. That’s me narrating (with a bit of a cold).

More animations and images can be found at nytimes.com/creaturecast and creaturecast.org, a project supported in part by the National Science Foundation.

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